Quantum Physics

Commerial quantum

Are Many Worlds and the Multiverse the Same Idea?
When physicists are asked about “parallel worlds” or ideas along those lines, they have to be careful to distinguish among different interpretations of that idea.

There is the “multiverse” of inflationary cosmology, the “many worlds” or “branches of the wave function” of quantum mechanics, and “parallel branes” of string theory. Increasingly, however, people are wondering whether the first two concepts might actually represent the same underlying idea. (I think the branes are still a truly distinct notion.) At first blush it seems crazy — or at least that was my own initial reaction. When cosmologists talk about “the multiverse,” it’s a slightly poetic term. The situation in quantum mechanics is superficially entirely different. These two ideas sound utterly different. Physical Theories, Eternal Inflation, and Quantum Universe, Yasunori NomuraThe Multiverse Interpretation of Quantum Mechanics, Raphael Bousso and Leonard Susskind The result is: multiverse-in-a-box.
Experiment. Experiment - Multimedia. How Cosmic Inflation Creates an Infinity of Universes [Video]

Arovas_Stern_2007. World line. In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime.

The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" (such as an orbit in space or a trajectory of a truck on a road map) by the time dimension, and typically encompasses a large area of spacetime wherein perceptually straight paths are recalculated to show their (relatively) more absolute position states — to reveal the nature of special relativity or gravitational interactions. The idea of world lines originates in physics and was pioneered by Hermann Minkowski. The term is now most often used in relativity theories (i.e., special relativity and general relativity). However, world lines are a general way of representing the course of events. The use of it is not bound to any specific theory. Usage in physics[edit] , upwards and the space coordinate, say horizontally.

Diffraction of water waves (opening)
<em>From Eternity to Here:</em> Book Club. From Eternity to Book Club: Chapter Eleven. Welcome to this week’s installment of the From Eternity to Here book club.

Part Three of the book concludes with Chapter Eleven, “Quantum Time.” Excerpt: This distinction between “incomplete knowledge” and “intrinsic quantum indeterminacy” is worth dwelling on.
Density matrix. Explicitly, suppose a quantum system may be found in state with probability p1, or it may be found in state with probability p2, or it may be found in state with probability p3, and so on.

The density operator for this system is[1] By choosing a basis (which need not be orthogonal), one may resolve the density operator into the density matrix, whose elements are[1] For an operator (which describes an observable. Einselection. In quantum mechanics, einselection, short for environment - induced superselection, is a name coined by Wojciech H.

Zurek[1] for a process which explains the phenomenon of wavefunction collapse and the emergence of classical descriptions of reality from quantum descriptions. Classicality is an emergent property induced in open quantum systems by their environments. Due to the interaction with the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable to entangling interaction with the environment, which in effect monitors selected observables of the system.

After a decoherence time, which for macroscopic objects is typically many orders of magnitude shorter than any other dynamical timescale,[2] a generic quantum state decays into an uncertain state which can be decomposed into a mixture of simple pointer states. In this way the environment induces effective superselection rules.
From Eternity to Book Club: Chapter Twelve. Welcome to this week’s installment of the From Eternity to Here book club.

Part Four opens with Chapter Twelve, “Black Holes: The Ends of Time.” Excerpt: Unlike boxes full of atoms, we can’t make black holes with the same size but different masses. The size of a black hole is characterized by the “Schwarzschild radius,” which is precisely proportional to its mass. If you know the mass, you know the size; contrariwise, if you have a box of fixed size, there is a maximum mass black hole you can possibly fit into it.

It’s not surprising to find a chapter about black holes in a book that talks about relativity and cosmology and all that. Black holes are important to our story for a couple of reasons. The other reason black holes are important, of course, is that the answer that Bekenstein and Hawking derive is somewhat surprising, and ultimately game-changing.